This interface defines the contract a value aggregator descriptor must
support. Such a descriptor can be configured with a Configuration
object. Its main function is to generate a list of aggregation-id/value
pairs. An aggregation id encodes an aggregation type which is used to
guide the way to aggregate the value in the reduce/combiner phrase of an
Aggregate based job.
The mapper in an Aggregate based map/reduce job may create one or more of
ValueAggregatorDescriptor objects at configuration time. For each input
key/value pair, the mapper will use those objects to create aggregation
id/value pairs.